1. Field of the Invention
The present invention relates to a capacitance-based sensor, which is embedded in a vehicle seat, and also relates to an occupant sensing system, which determines an occupant state of the seat based on an output of the capacitance-based sensor.
2. Description of Related Art
One type of occupant sensing system includes a capacitance-based sensor and an occupant sensing electronic control unit (ECU). The capacitance-based sensor senses a disturbance in a weak electric field generated by an electrode and outputs the sensed result as the corresponding electric current or electric voltage (see, for example, Japanese Unexamined Patent Publication No. H11-271463).
Specifically, in a state of an empty seat where no occupant is present in the seat, the air is interposed between two electrodes of the capacitance-based sensor. In another state where a child restraint system (CRS) is installed in the seat, the CRS is interposed between the two electrodes of the capacitance-based sensor. In yet another state where an occupant is present in the seat, a body of the occupant is interposed between the two electrodes of the capacitance-based sensor.
Here, a dielectric constant of the air is about 1. A dielectric constant of the CRS is about 2 to 5 although it may vary depending on a material of the CRS. Furthermore, a dielectric constant of the human body is about 50. As illustrated above, the dielectric constant of the air, the dielectric constant of the CRS and the dielectric constant of the human body differ from one another. Thus, a capacitance between the two electrodes of the capacitance-based sensor varies depending on the type of the interposed object, which is interposed between the two electrodes.
Based on the differences in the capacitance, the occupant sensing ECU performs the occupant determination. Specifically, the occupant sensing ECU determines whether the seat is empty, whether the CRS is installed in the seat and/or whether an adult is present in the seat. An air bag ECU determines enablement/disablement of deployment of an air bag based on the result of the determination of the occupant sensing ECU. Specifically, in the state of the empty seat or in the state of the CRS installed in the seat, the deployment of the air bag is disabled. On the other hand, in the state where the adult is present in the seat, the deployment of the air bag is enabled.
The capacitance, which is measured with the capacitance-based sensor in the state where the occupant is present in the seat, includes the capacitance, which is generated upon seating of the occupant in the seat, and the capacitance, which has been present since the empty state of the seat. This point will be described with reference to FIG. 34. FIG. 34 is a diagram schematically showing a circuit structure of a previously proposed capacitance-based sensor. In FIG. 34, “Cb” is a capacitance between an electrode 120 and a vehicle ground, i.e., GND (or a seat frame that is grounded to the vehicle ground) through a human body, and “Co” is a capacitance (an empty seat capacitance), which has been present between the electrode 120 and the vehicle GND since the empty state of the seat. An electric current sensing device (an electric current sensing circuit) 131 measures the capacitance as a sum of the capacitance Cb and the capacitance Co. The capacitance (the empty seat capacitance) Co between the occupant sensing electrode 120 and the vehicle GND constitutes a relatively large ratio in the entire capacitance (the sum of the capacitance Cb and the capacitance Co), and variations in the empty seat capacitance Co directly causes variations in the entire capacitance to deteriorate the level of the accuracy in the occupant determination. Furthermore, the adjustment of the variation in the measured capacitance of the capacitance-based sensor in each vehicle causes a large increase in the factory work load.
Furthermore, with reference to FIG. 35, in the case of the occupant sensing system recited in Japanese Unexamined Patent Publication No. H11-271463, an occupant sensor 514 is provided on a top surface of a seat bottom 510 of a passenger seat. The occupant sensor 514 includes a first electrode 512 and a second electrode 513. An electric field is created between the first electrode 512 and the second electrode 513. When an occupant is seated on the seat bottom 510, a capacitance between the first electrode 512 and the second electrode 513 changes to cause a change in the electric current between the first electrode 512 and the second electrode 513. By measuring the change in this electric current, presence of the occupant on the seat bottom 510 is sensed.
However, there is provided only the single set of electrodes 512, 513. Thus, it is not possible to cover various seating patterns of the occupant. For example, when the occupant is displaced from the single set of electrodes 512, 513, the presence of the occupant on the seat bottom 510 cannot be effectively sensed.